A. Field of the Invention
The present invention relates to a silicon carbide semiconductor element, a method of manufacturing the same, and a silicon carbide device.
B. Description of the Related Art
Silicon carbide (hereinafter also referred to as SiC) is thermally, chemically and mechanically stable, and is expected to be applied to a wide range of industry fields including light emitting elements, high frequency devices, and power semiconductor devices. High withstand voltage MOSFETs using SiC, for example, have an advantage of lower on-resistance than high withstand voltage MOSFETs using silicon (Si). Schottky diodes using SiC have an advantage of lower forward voltage drop than Schottky diodes using silicon.
Intrinsically, on-resistance and switching speed of power devices are in a trade-off relation. Nevertheless power devices using SiC can achieve low on-resistance and high switching speed at the same time. In order to achieve low on-resistance or high switching speed, it is necessary to reduce contact resistance at an ohmic contact.
Recently, there is a technique widely employed for forming a low resistance ohmic contact in an n-type SiC region, in which an ohmic electrode structure is formed by depositing an electrode film, and the ohmic electrode structure is then heat treated at a high temperature in the range of 800° C. to 1,200° C. (See for example, Japanese Patent No. 3303530, and Japanese Unexamined Patent Application Publication Nos. 2002-175997 and H08-064801. The electrode film is, for example, a film of nickel, tungsten or titanium. Nickel, in particular, used for an electrode film, has attained a practical contact resistance value in an order of 10−6Ω cm2 at an ohmic contact. So, an ohmic contact using nickel is very promising ohmic contact.
FIG. 11 shows a result of elemental analysis along the depth direction of a conventional ohmic contact using nickel obtained by means of X-ray photoelectron spectroscopy. In FIG. 11, the ordinate represents the content of the elements and the abscissa represents the sputtering time (arbitrary unit). In order to form an ohmic contact shown in FIG. 11, a typical nickel film first is deposited on a SiC substrate by a sputtering method. Then, a heat treatment is conducted in a vacuum (not higher than 5×10−4 Pa) at 1,000° C. for 5 minutes. The heat treatment at the high temperature forms a conductive thermal reaction layer (a nicked silicide film) of a Ni—Si—C mixture on the nickel film surface. In the surface region of the nickel silicide film, a large amount of carbon precipitates that are diffused from the SiC substrate and the surface is virtually covered with the carbon.
When a wiring conductor element such as aluminum film is connected to an electrode film of a nickel film with precipitates of carbon thereon, a problem of increase in contact resistance arises between the electrode film and the wiring conductor element. There is an additional problem that the wiring conductor element is liable to detach.
A method to prevent carbon from precipitating has been proposed (Japanese Patent No. 3646548, for example) in which an ohmic electrode is formed of an alloy composed of nickel and another metal that easily forms a carbide, and a nickel film is formed on the film of this alloy. Another method has been proposed (Japanese Unexamined Patent Application Publication No. 2006-202883, for example) in which an ohmic electrode is composed of a nickel silicide film, a first nickel film, a titanium film, a film of alternately formed nickel and silicon, and a second nickel film laminated in this order.
However, Japanese Patent No. 3646548 and Japanese Unexamined Patent Application Publication No. 2006-202883 fail to mention a technology to connect a wiring conductor element such as an aluminum film to the electrode film of such as nickel. Consequently, it is a problem with these methods of these two documents that a wiring contact element may fail to be connected to a nickel film on which a silicon oxide film is formed. The technology disclosed in Japanese Unexamined Patent Application Publication No. 2006-202883 has an additional problem of complicated manufacturing process and high cost due to a multiple of laminated layers.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.